1. Field of the Invention
The present invention relates to a wrist mechanism of an industrial robot having three wrist elements rotatably supported by an arm of the robot relative to each other around respective rotation axes which are mutually inclined, which leave a hollow space inside thereof free for passage of supply cables and pipes which are to be connected to a tool being secured to the robot wrist.
2. Description of the Related Art
Such wrist mechanisms of the industrial robots having two or three wrist elements, which leave a hollow space inside thereof free for passage of supply cables and pipes, are disclosed, for instance, in U.S. Pat. No. 5,549,016. This patent discloses a robot wrist having two reduction units carried in the robot wrist to drive two wrist elements and a reduction unit to drive the first wrist element or entire robot wrist being carried in an arm of a robot. Recently, many supply cables and pipes are requested to house spaces within the inside the robot wrist for supplying electricity and fluid, such as pressurized oil or air etc. to a tool being secured to the robot wrist. To meet such a request, for example, U.S. Pat. No. 6,014,909 discloses a robot wrist having hollow drive tubes and hollow bevel gears for transmitting rotational movements leaving a hollow space inside thereof free for passage of supply cables and pipes. For transmitting rotational movements to wrist elements, U.S. Pat. No. 6,014,909 uses chains of gears as the final input transmitting members to each of the wrist elements. In cases where chains of gears are used as final input transmitting members, a difficulty arises in that positioning the wrist elements becomes unstable due to backlashes in the gears. Further, the positioning becomes more unstable due to flank wear of the gears suffering from high torques applied thereon, especially when robots are handling a heavy load, such as 100 kg. Another difficulty in positioning the wrist elements arises from a large amount of reflected or followed rotational movements of a third wrist element caused by rotational movements of first and second wrist elements, that is, since the first, second and third hollow drive tubes are independently supported by an arm of a robot adapted to drive first, second and third wrist elements relative to each other, a rotational movement of the first wrist element effects or follows those of the second and third wrist elements, as well as a rotational movement of the second wrist element effects or follows that of the third wrist element.
To eliminate these deficiencies, applicant proposed, in pending undisclosed or unpublished Japanese patent application No. 11 (1999)-31418, a robot wrist as shown in FIG. 3 of this application. However, more sufficient spaces for passage of the supply cables and pipes, and more stable positioning of wrist elements to reduce play between power transmitting elements are desirable.
An object of the present invention is to provide a wrist mechanism of an industrial robot which leaves a sufficient hollow space inside thereof free for passage of supply cables and pipes which are to be connected to a tool being secured to the wrist mechanism, wherein difficulties in positioning the wrist elements becoming unstable due to backlashes and flank wear in final input transmitting gears are eliminated and accuracies of controlling thereof are increased.
Another object of the present invention is to provide a wrist mechanism of an industrial robot wherein amounts of reflected rotational movements of second and/or third wrist elements affected by rotational movements of first and second wrist elements are made small, thereby the controllings of the positionings of positionings of wrist elements become easy and accuracies of controllings thereof are increased.
These and other objects are achieved according to a first aspect of the present invention by a wrist mechanism of an industrial robot comprising: a first ring-shaped hollow reduction unit secured to an arm of the robot, a first hollow drive tube and a second ring-shaped hollow reduction unit secured to an output shaft of the first reduction unit, respectively, a second hollow drive tube and a third ring-shaped hollow reduction unit secured to an output shaft of the second reduction unit, respectively, and a third hollow drive tube secured to an output shaft of the third hollow reduction unit rotatably around a first axis, further, said first, second and third hollow drive tubes being connected to the first, second and third wrist elements, respectively, to rotate each of the wrist elements relative to each other.
By such arrangements, by using the first, second and third ring-shaped hollow reduction units connect to the first, second and third hollow drive tubes, respectively, to rotate each of the first, second and third wrist elements connected thereto relative to each other, a sufficient hollow space is secured inside thereof free for passage of supply cables and pipes which are to be connected to a tool being secured to the third wrist element. Further, since no chains of gears are used as final input transmitting members to each of the wrist elements, amounts of backlashes and flank wear are eliminated, thereby difficulties in positioning the wrist elements becoming unstable due to backlashes and flank wear in the gears are eliminated, and accuracies of controllings thereof are increased.
Similarly, since amounts of reflected rotation movements of the second and/or third wrist elements affected by the rotational movements of the first and second wrist elements are reduced within each fraction of the reduction ratios of each of the reduction units, the resulting amounts of their reflected rotational movements are small, thereby positioning the wrist elements becomes easy and accuracies of controllings thereof are increased.
The above objects are also achieved according to a second aspect of the present invention by a wrist mechanism of an industrial robot comprising: a first, second and third hollow drive tubes rotatably supported by an arm of the robot relative to each other coaxially around a first axis, said second and third hollow drive tubes being connected to second and third hollow connecting members, respectively; a first hollow wrist element secured to and supported by the first drive tube rotatably around the first axis; a second hollow wrist element secured to a hollow second wrist element supporting member being supported by the first wrist element rotatably around a second axis inclined with respect to the first axis; said second wrist element supporting member being connected with the second connecting member via a universal joint which comprises, two pairs of radial shafts each pair thereof being rotatably secured to each members, respectively, on each two positions crossing with a diametrical line passing through a point lying on each axes of the members at even intervals from an apex on which the axes of the members cross, two tublar rings respectively secured to each pair of the radial shafts, two pairs of axial bearings rotatably secured to the two tublar rings on their respective portions 90xc2x0 circumferentially offset from the radial shafts, respectively, and a pair of connecting bars both ends of which being secured to the two pairs of axial bearings, respectively, thereby the connecting bars connecting two tublar rings together, thereby connecting the second connecting member and the second wrist element supporting member; and a third hollow wrist element secured to a hollow third wrist element supporting member being supported by the second wrist element rotatably around a third axis inclined with respect to the second axis; further, said third wrist element supporting member is connected with the third hollow connecting member via a forth hollow connecting member rotatably supported by the second wrist element supporting member, via each of the universal joints on both sides of the forth connecting member.
By such arrangements, since no chains of gears are used as final input transmitting members to each of the wrist elements, difficulties in positioning the wrist elements becoming unstable due to backlashes and flank wear in the gears are eliminated, and accuracies of controllings thereof are increased. And, since power transmissions are performed by using universal joints having radial shafts, two tubular rings and connecting bars which have tougher strengths than those of a chain of gears, a more compact and tougher wrist mechanism is provided which is able to handle a high load secured to the wrist mechanism. Further, because the connecting bars of the universal joints rotate substantially at a uniform speed, and do not rotate unevenly, reflected rotational movements of the second and/or third wrist elements affected by the rotational movements of the first and second wrist elements are made to rotate substantially at a uniform speed, thereby positioning the wrist elements becomes easy.